The existing CHECK assumed that the source and destination could not
have the same buffer, but they actually can as long as the data
ranges do not overlap within the buffer. Change the check to look for
this more relaxed condition instead.

Moved the check outside of the memcpy case as well, given that it
should also apply for the slower, element-by-element copy as well.

Also use JSTypedArray::element_size() to get the element size instead
of the helper on the FixedTypedArrayBase. This lets us change that
helper back to private again.

Bug: chromium:717022

Change-Id: I2eca1df1e87444c5db397e0b7cf686cefe67d29c
Reviewed-on: https://chromium-review.googlesource.com/493147
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45035}